Performance analysis of a square lattice PCF based surface plasmon resonance sensor

Abstract

In this dissertation, photonic crystal fiber (PCF) based surface plasmon resonance (SPR) sensor has been investigated. Conventionally, hexagonal lattice PCF with modified structure is often used in SPR sensing probe. Being simpler in structural geometry than hexagonal lattice PCF, square lattice PCF is investigated for its suitability in SPR sensing system. Square lattice structure is modified to construct multiple cores with centrally located analyte channel. Designing the sensor with only one analyte channel has reduced the amount of gold required for coating. The amount of analyte required for effective detection and also the co-channel interference from neighboring channel is expected to be reduced. Use of multiple cores has effectively increased the overlap between core guided light and surface plasmons, resulting better sensitivity. Besides, performance parameters like sensitivity, detection accuracy or signal-to-noise ratio (SNR), sensing range and detection linearity has been evaluated. Factors contributing to the refractive index sensitivity like metal coating width, use of different metal for coating, bimetallic layer, different doping concentration etc. are explored systematically through design, simulation, data acquisition, data interpretation and analysis. The proposed design successfully contributed in the excitation of SPPs and confirmed the suitability of square lattice PCF for SPR sensing system. The proposed design with gold coating yielded a sensitivity of 7432 nm/RIU in the sensing range of 1.43 to 1.50 refractive index. It has also resulted in achieving a SNR of 1.202. The detection linearity obtained is also very high (0.99911). The suitability of other metal such as silver and copper has also been studied. Silver and copper did not show better sensitivity than gold but exhibited promise in enhancement of SNR. It is also found that silver and copper cannot be used without protective coating because of oxidation, corrosion and chemical reactivity. As sensitivity also depends on the core material, different doping concentration has also been used to tune the sensitivity. Doping concentration of 13.5% GeO2, 9.1 % P2O5 and 5.2 % B2O3 has been used separately with pure silica. Doping with 13.5% GeO2 yielded better sensitivity than pure silica with a value of 8411 nm/RIU, while other two dopants resulted less sensitivity than pure silica. To utilize the potential of silver to produce better SNR, the sensitivity and SNR of the sensor is further tailored by the use of bimetallic layer consisting gold and silver. Investigation with bimetallic layer yielded a sensitivity of 13180 nm/RIU. The SNR also got enhanced from a value of 1.202 to 1.32. With these findings, this study offered a modified approach for PCF based SPR sensor probe, encompassing a promise of enhanced performance and simplicity in design and fabrication.